1. Field of the Invention
The invention relates to a process for working-up hydrogen sulfide-containing gases in which the sulfur constituents of the said gases are converted to elemental sulfur in a sulfur recovery unit.
2. Description of the Prior Art
In general, feedstocks for sulfur recovery units are hydrogen sulfide-containing gases produced in the regeneration step of gas purification processes. These gas purification processes, which are required in order to reduce the sulfur constituents (in most cases mainly hydrogen sulfide) of industrial gases, such as refinery gas, natural gas or synthesis gas produced by the partial combustion of hydrocarbons, to acceptably low levels prior to their further use, usually involve absorbing the sulfur components of the gases in a liquid absorbent which is then regenerated to give hydrogen sulfide-rich gases. These latter gases are then passed to a sulfur recovery unit, elemental sulfur being produced and removed. The reaction off-gases of such a unit are then preferably further treated in order to reduce the sulfur content of the gases to a very low level before the gases are finally discharged to the atmosphere.
In the sulfur recovery unit the hydrogen sulfide is converted to elemental sulfur by the well-known Claus reaction. Sulfur recovery units of the Claus type or Claus plants are widely used in industry and are well-established.
In the following description, "Claus reaction" refers to the reaction between hydrogen sulfide and sulfur dioxide in which elemental sulfur and water are produced. By "sulfur recovery unit of the Claus type" or "Claus plant" is meant a plant for carrying out the Claus reaction, incorporating a thermal zone in which hydrogen sulfide is partially combusted to produce sulfur dioxide in the correct proportion which then reacts with the unburned hydrogen sulfide to give sulfur and water, the sulfur then being condensed and recovered, followed by one or more catalytic zones in which the same reaction is further promoted by means of a suitable catalyst and additional sulfur is recovered.
The process carried out in a Claus plant is sometimes referred to in literature as modified Claus process.
Apart from hydrogen sulfide, the above-mentioned industrial gases often contain other sulfur constituents such as carbonyl sulfide and other organic sulfur compounds, e.g., mercaptans, which must also be removed before these gases are used. Accordingly, a suitable liquid absorbent is used which removes all sulfur constituents present. However, the said industrial gases usually also contain carbon dioxide. The liquid absorbents applied in the gas purification processes for the removal of hydrogen sulfide are generally also good absorbents for carbon dioxide. For the removal of carbonyl sulfide by means of absorption, a solvent is in practice used which is an efficient absorbent for carbon dioxide. Consequently, regeneration of the fat liquid absorbent yields hydrogen sulfide/carbon dioxide-containing gases which have then to be processed in the Claus plant.
A process for reducing the sulfur content of off gases from a Claus plant by conversion of the sulfur content of the gases to H.sub.2 S, absorption of the H.sub.2 S, recovery thereof and recycle to the Claus plant is the subject of U.K. Patent Specification No. 1,356,289. Said process is readily operable so long as the gases recycled to the Claus plant have a low carbon dioxide content (up to about 15% by volume) and a relatively high hydrogen sulfide content (above 50% by volume). However, if the gas fed to the Claus plant contains very high amounts of carbon dioxide, a Claus plant must be specially designed to handle such a feed. Such a plant is larger and less efficient then a plant designed for a feed with low CO.sub.2 content. If, moreover, the hydrogen sulfide content of such hydrogen sulfide/carbon dioxide-containing gases is less than 40% by volume, the Claus plant is difficult to operate with respect to the thermal zone and special measures have to be taken when combusting part of the hydrogen sulfide to produce the necessary sulfur dioxide required for the Claus reaction. These operational difficulties mainly arise from the fact that the required temperatures for the combustion of hydrogen sulfide cannot be reached in the thermal zone.
Nowadays, industrial gases have to be purified with gas purification processes which yield hydrogen sulfide/carbon dioxide-containing gases which have even less than 20% by volume of hydrogen sulfide whereas the carbon dioxide content is above 20% by volume. Such industrial gases may also result from processes for the conversion of solid fuels, such as coal, to low sulfur gas or liquid fuel.